It sounds too good to be true – A product that prevents weeds and grass from spouting in garden beds and containers. That is exactly what a pre-emergent does. Timing is critical. If applied too early, the product is washed away by rains before it has time to do its magic. If applied after the weeds have already sprouted, it is useless. Strictly speaking, pre-emergent herbicide does not actually prevent germination of weed seeds. Instead it creates a chemical layer in the soil by inhibiting a particular enzyme the weeds need to grow.

Weather varies from year to year and from location to location. Pre-emergents are effective if used just before weeds begin to germinate. The proper time to apply a pre-emergent cannot be a specific assigned date. Instead, follow this reliable trigger: Apply when Forsythia is in bloom. Follow package directions exactly. Some call for the application of water after application; others say to incorporate the product by scratching it into the surface. For those that direct the use of water, use overhead sprinkling to dissolve the granules. A soaker hose or drip tape is ineffective. Heavy rain can dissolve the granules, but a light rain stretching over several days will only leave a very thin “no grow” layer. Under normal conditions, the herbicide will be effective 3 to 5 months. Continued heavy rains or frequent irrigation will reduce the concentration of the herbicide and hence shorten its effectiveness.

Once the product has been applied, avoid disturbing the soil and the chemical barrier. This means no digging, raking, dragging a hose across a treated area, or even jiggling a treated container enough to disturb the soil surface. Most weed seeds germinate in the top half-inch of soil. There are few one-size-fits-all product. Some work to prevent grasses; others target broadleafs like dandelions, clover, or chickweed. If your beds are infested with both, you should look for a product marked “broad spectrum.” Otherwise, you may need to apply two separate products.

Do not apply grass seed after using a pre-emergent or your grass seed will not sprout and grow successfully. And those “Weed and Feed” combination products? Take a pass. The correct time to apply a weed-prevention product is not the correct time to fertilize.

Most pre-emergents are not for use in vegetable gardens. There are a couple that can be used around edibles. Be sure to check the label. I use several brand names in the Mary Snoddy garden. Here’s a tip for easy application: Wash and dry an empty plastic quart container, such as that from mayonnaise. Use a drill bit to bore holes through the plastic lid, creating a giant “salt shaker.” Fill the container with your herbicide and sprinkle it on. This home-made applicator allows for controlled, consistent application.